This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to transmitters having a slack take-up device for remote sensing apparatuses.
In one type of process control system, a pressure transmitter is used to remotely monitor the pressure of a process fluid. The pressure transmitter includes circuitry that conditions an electrical output of a pressure sensor and transmits it to a remote location where it can be monitored as representing the magnitude of the pressure. Remote seals, or remote diaphragm assemblies, are often used to distance the pressure transmitter from hazardous measurement environments, or for linking the pressure transmitter with inconveniently located process fluids. For example, remote seals are often used with corrosive or high temperature process fluids such as in chemical plants or oil refineries. Typically, in those situations, a mechanical remote seal having a diaphragm assembly and a capillary tube is used to relate the pressure transmitter to the process fluid through a hydraulic fill fluid, while the pressure transmitter is located a safe distance away. The flexible diaphragm isolates the process fluid from the fill fluid used in the capillary tube. As the diaphragm flexes, the incompressible fill fluid translates pressure change through the capillary tube to a diaphragm located in the pressure transmitter. Deflection of a pressure transmitter diaphragm is transmitted through another fill fluid to a pressure sensor, which produces a signal relating to the pressure of the process fluid.
Capillary tubes can extend tens of meters in order to couple the pressure transmitter with the process fluid. Because of costs and difficulty associated with customizing the length of the capillary tube, remote seal assemblies are typically made available with stock lengths of capillary tube. Often times, however, the remote seal comes with an excessive length of capillary tube for some applications. Also, for differential pressure measurement in balanced configurations, where two remote seals are used with equal lengths of capillary tubing in order to equalize back pressure, one of the capillary tubes is typically longer than necessary for the application. Due to the sensitive nature of the sensors and remote seals, which are pre-filled with a precise amount of fill fluid at the factory, it is impractical to adjust the length of capillary tubes in the field. Thus, it becomes necessary in field environments to deal with excess lengths of capillary tube in order to ensure their security and pressure transmission performance. However, it is often the case that the capillaries are jumbled up or stashed such that they may easily become crimped, cut or otherwise compromised, which also affects their pressure transmission capabilities. As such, there is a need to eliminate the problems associated with excessive capillary lengths in remote seal assemblies.